Conventional servovalves convert relatively low power electrical control input signals into a relatively large mechanical power output. Certain servovalves, such as fuel control servovalves utilized in aircraft, use aircraft engine fuel as the pressurized fluid that controls a corresponding fluid motor. For example, during operation pressurized aircraft fuel enters the fuel control servovalve and, based upon the control input signals, drives the fluid motor to operate nozzles and other variable-geometry elements associated with the aircraft.
FIG. 1 illustrates an example of a conventional fuel control servovalve 10, such as a jet pipe servovalve. The typical jet pipe servovalve 10, for example, includes a housing 12 having a sleeve 24, a spool 26 disposed within the sleeve 24, a motor 21 coupled to the spool 26 via a feedback spring 23, a fluid jet 25 associated with the motor 21, and a fluid receiver 27. When the motor 21 receives an input signal, the motor causes the spool 26 to meter fluid flow between a pressurized fluid source 20 and a hydraulic or fluid motor 33 coupled to the servovalve 10.
Servovalve spools 26 typically have lands 28 that meter fluid flow within the servovalve 10 between the fluid source 20 and the fluid motor 32. Each land 28 of the spool 26 has an outer diameter that is smaller than an inner diameter of the sleeve 24. The lands 28, therefore, form a gap or clearance with an inner surface of the sleeve 24 that allows the spool 26 to translate within the sleeve 24 and minimizes potential binding of the spool 26 within the sleeve 24 during actuation. Typically, spools 26 have several lands 28 arranged in pairs or sets. Each pair of lands 28 of a conventional spool 26 defines a control channel or control groove 32 oriented between adjacent lands 28-1, 28-2 of the pair. The control channel 32 helps to equalize pressure within the sleeve 24 and spool 26 allowing the spool 26 to actuate relatively smoothly within the sleeve 24.
Assume the servovalve 10 is configured in a pressurized state where a fluid flows from the fluid source 20, through a conduit 22, and into the sleeve 24. When the spool 26 positions in a null or closed position within the sleeve 26, as shown in FIG. 1, a pair of lands 28, along with the associated control channel 32, cover an associated, rectangularly shaped port 30 oriented between the fluid source 20 and the fluid motor 33. In the null position, the lands 28-1, 28-2 of the spool 26 (e.g., outer edges of the lands 28-1, 28-2) prevent or minimize fluid flow between the fluid source 20 and the fluid motor 33 via the port 30.
During operation, the motor receives a control signal from a control signal source and, in response, causes the spool 26 to actuate or move within the sleeve 24 to an open position such that the pair of lands 28 offsets from (e.g., uncovers) the rectangularly shaped port 30. For example, when the servovalve 10 orients the spool 26 in a null position, the jet 25 impinges receiver openings 29 defined by the fluid receiver 27. When the motor 21 receives an input signal, the motor 21 diverts the jet 25 such that the jet 25 substantially impinges one of the receiver openings 29, thereby increasing a pressure within a channel 31 (e.g., one of a pair of channels 31) associated with the impinged receiver opening 29. The pressure differential on the spool 26 causes the spool 26 to actuate in the sleeve 24 to an open position. In the open position, the set of lands 28 meters an amount of fluid flowing between the fluid source 20 and the fluid motor 33 to control positioning or movement of a load coupled to the fluid motor 33. As the spool 26 moves in response to the input signal, the spool generates an opposing torque on the feedback spring 23 that returns the fluid jet 25 to a substantially centered position and creates a force balance across the spool 26, thereby bringing the spool 26 to a position of positional equilibrium.